Solar cells made of single- or multi-crystalline silicon are usually provided with a dielectric coating on a front side (i.e. the light incident side) in order to lead the incident light effectively to the semiconductor layer. Such a dielectric coating is often referred to as anti-reflection coating (ARC) film.
The performance of a solar cell is largely influenced by the degree of suppression of recombination of the photo-generated carriers at the interface between the semiconductor layer and the ARC film. Suppression of recombination of the photo-generated carriers is normally realized using what is called surface passivation.
As an ARC film for multi-crystalline silicon solar cell, a silicon nitride film is often used because it has a good anti-reflecting effect and a sufficient surface passivation effect can be expected. It is also used for single crystalline silicon solar cells for the same reason. Alternatively, a thermal oxide film is used in which case more effective surface passivation can be expected than by silicon nitride.
Normally, a thermal oxide film with sufficient surface passivation requires a high temperature process (approximately 1000° C.), which will deteriorate the efficiency of the solar cells. Additionally, the refraction index of thermal oxide film (1.45) is too low for the proper ARC for silicon solar cells.
In crystalline silicon solar cells, a back surface field (BSF) layer is usually formed by coating and alloying by heat treatment of an aluminium paste on the back side. The thickness of the crystalline silicon solar cell will most certainly decrease in the future because of a shortage of the silicon feedstock. This will lead to a worse effectiveness of the BSF layer since it will bend the thin substrate and will also lower the internal reflection at the back side. Nowadays, in order to replace the BSF layer, a dielectric film, such as a silicon nitride film, or a thermal silicon oxide film is adopted with a partly removed area for back side electrodes. As mentioned above, a silicon nitride film can provide a good passivation effect and a thermal oxide film can be even better. Additionally, these dielectric films can enhance the internal reflection at the back side of the solar cells as compared to aluminium BSF.
Requirements for a dielectric film deposited on a semiconductor substrate for a crystalline silicon solar cell are:                formable at relatively low temperature        high passivation effect        anti-reflection effect when formed at the front side        anti-reflection effect or enhancement of the internal reflection when formed at the back side.        
For such a dielectric film, when used for optimal anti-reflection effect, the refractive index should be lower than that of silicon (3.3) and higher than that of packaging resin or cover glass (1.4˜1.6). Silicon nitride film can satisfy most of the conditions above, but its passivation effect is inferior to that of a thermal oxide film. A thin thermal oxide may be inserted between the silicon and the silicon nitride, to satisfy the conditions described above, without reducing either optical effect of anti-reflecting at the front or enhancing internal reflection at the back in spite of low refractive index (1.45) of thermal oxide. A thermal oxide film with sufficient surface passivation requires a high temperature process (approximately 1000° C.), which will deteriorate the efficiency of the solar cells. However, forming a sufficiently thin (<70 nm) thermal oxide with a good surface passivation and under good control is very difficult. One possibility is to provide a thermal oxide film on the silicon substrate and then to thin the thermal oxide film by way of etching, but in that case it is impossible to create a thermal oxide film with a uniform thickness. Even though thermal oxidation at lower temperatures (around 800° C.) can form a thin oxide film to some extent, its surface passivation effect is normally low and sometimes even inferior to that of a silicon nitride film.